I'd always read that the methane production of cattle was due mostly to mankind meddling in their diet.  They were not designed to handle a diet of potatoes, sugar beets, corn and grain, they were designed for grass.  We feed them the other stuff to fatten them up for market.   If I remember correctly the standard feedlot diet disagrees with them so much, they have to have a constant supply of anti-gas medication added to their feed (bicarbonate of soda?).

http://en.wikipedia.org/wiki/Feedlot